The present invention relates to a method of forming a semiconductor device, and more particularly to a method of forming an opening in an insulation film over a semiconductor substrate.
Formation of opening portions in insulation film over a semiconductor substrate is essential process for forming various semiconductor devices.
A typical one of the conventional methods of forming a semiconductor device will be described with reference to FIGS. 1A through 1E, which are fragmentary cross sectional elevation views illustrative of semiconductor devices in sequential steps involved in the conventional method of fabricating a bipolar transistor.
With reference to FIG. 1A, a photo-resist is applied onto a top surface of a silicon substrate 1 to form a photo-resist film 21A. An opening is formed in the photo-resist film 11A so that the opening is positioned over a base formation region of the silicon substrate 1 to form a photo-resist pattern 11A. The photo-resist pattern 11A is used as a mask for ion-implantation of a p-type impurity into the base formation region of the silicon substrate 1 whereby a p-type base region 2 is formed in the base formation region of the silicon substrate 1.
With reference to FIG. 1B, after the photo-resist film 11A has been removed, then a silicon dioxide film 12 is entirely grown over the top surfaces of the silicon substrate 1 and the p-type base region 2. Further, a silicon nitride film 13 is then entirely grown over the top surface of the silicon dioxide film 12 because the silicon nitride film 13 is hard and serves as a barrier layer against contamination of impurity.
With reference to FIG. 1C, a photo-resist film 11B is provided over the top surface of the silicon nitride film 13. Opening portions are then formed in the photo-resist film 11B so that the opening portions are positioned over an emitter formation region and a base contact formation region in the p-type base region 2. The photo-resist film 11B is used as a mask for subsequent dry etching to the silicon nitride film 13 to form openings in the silicon nitride film 13. Subsequently, the silicon dioxide film 12 is subjected to a wet etching thereby to form openings 14A and 14B over the emitter formation region and the base contact formation region in the p-type base region 2. The wet etching is carried out to prevent the p-type base region 2 in the silicon substrate 1 from receipt of any damage. By contrast to the dry etching as an anisotropic etching, the wet etching is an isotropic etching, for which reason the silicon dioxide film 12 is subjected to a side etching namely etching is progressed not only in a vertical direction but also in a horizontal direction.
With reference to FIG. 1D, after the photo-resist film 11B has been removed, then a polysilicon film 17 is entirely grown over the silicon substrate so that the polysilicon film extends over the silicon nitride film 13 and the emitter formation region and the base contact formation region in the p-type base region 2 as well as in the openings 14A and 14B. An ion-implantation of an n-type impurity into the polysilicon film 17 is carried out to form an n-type impurity doped polysilicon film 17. A photo-resist film is further entirely provided over the impurity doped polysilicon film 17. An opening is then formed in the photo-resist film so that the opening is positioned over the emitter formation region. The photo-resist film is used as a mask to selectively remove the n-type impurity doped polysilicon film 17 so that the n-type impurity doped polysilicon film 17 remains to extend over the emitter formation region of the p-type base region 2 of the opening 14A and within the opening 14A as well as extends on a part of the silicon nitride film 13 in the vicinity of the opening 14A. The photo-resist film has been removed, before the silicon substrate 1 is then subjected to a heat treatment to cause a thermal diffusion of the n-type impurity doped in the polysilicon film 17 into a shallow region of the emitter formation region of the p-type base region 2 whereby an n-type emitter region 18 is formed in the shallow region in the p-type base region 2.
With reference to FIG. 1E, titanium and platinum films are entirely formed acting as conductive and barrier layers. A photo-resist film is moreover provided entirely and then openings are formed in the photo-resist so that the openings are positioned over the emitter formation region and the base contact formation region. The photo-resist film is then used as a mask for gold plating to selectively form a gold plating film 20A over the polysilicon film 17 as well as selectively form a gold plating film 20B in the opening 14B and over a part of the silicon nitride film 13 in the vicinity of the opening 14B. The gold plating films 20A and 20B serve as emitter and base electrodes. Unnecessary parts of the barrier layers are then removed.
It should, however, be noted that cavities 19 are formed between the polysilicon film 17 and the silicon dioxide film 12 as well as between the gold plating film 20B and the silicon dioxide film 12 because the silicon dioxide film 12 has been side-etched in the past wet etching. In a post cleaning process, moisture may enter into the cavities 19. As a result, expansion of a portion around the cavities 19 may appear or crack may be formed in the vicinity of the cavities 19. This results in the drop of reliability of the bipolar transistor.
If, however, in order to avoid the above first problem, the dry etching is, in place of the wet etching, carried out to form the openings 14A, then the surface of the p-type base region receives a certain damage by the dry etching.
The above conventional method of forming the bipolar transistor is engaged with the second problem as follows. As described above, titanium and platinum conductive and barrier layers are formed before the photo-resist film with openings as acting as the mask is formed on the barrier layers for subsequent formation of the gold-plating layer acting as the electrodes. Unnecessary parts of the barrier layers are then removed. If the barrier layer is required to be thick, this results in a required long time plating process whereby a probability of permeation of the plating solution into an interface between the barrier layer and the photo-resist film. As a result, the adhesion between the barrier layer and the photo-resist film becomes weak. The reliability of the electrode layer is thus dropped.
In the above circumstances, it had been required to develop a novel method of forming a semiconductor device free from the above first and second problems.